In commonly assigned U.S. Pat. No. 4,968,925 of Rik W. A. A. De Doncker, issued Nov. 6, 1990, a universal field-oriented (UFO) controller is described which allows for field-oriented control of induction machines in an arbitrary flux reference frame. The principle of the UFO controller applies to both direct and indirect field orientation. In practice, this allows for the integration of six different field orientation schemes into a single control, i.e., direct and indirect field orientation in rotor flux, air gap flux and stator flux reference frames. In operation of the UFO controller, a synchronous reference frame is selected by setting the effective stator-to-rotor turns ratio to a predetermined value corresponding thereto. Transitions between reference frames are accomplished by changing the turns ratio. The result is complete decoupling of torque and flux in a flexible, simple and robust drive that is relatively insensitive to machine parameters. U.S. Pat. No. 4,968,925 is incorporated by reference herein.
U.S. patent application Ser. No. 87/769,692 of R. W. A. A. De Doncker, cited hereinabove, describes a high-speed flux feedback system for a UFO controller which quickly and accurately provides the amplitude and angle components of flux for use in a field-oriented control scheme. Flux feedback according to Ser. No. 07/769,692 results in tuned operation of an induction machine drive at the commanded flux and corresponding slip frequency unless errors are introduced by, for example, the current regulator and/or inverter. Such errors may cause the torque produced by the motor to differ from the commanded torque. Therefore, it is desirable to provide a torque feedback loop for a UFO controller in order to maintain robust torque control over a wide speed range.